High-strength steel sheet

ABSTRACT

A high-strength steel sheet has high stretch flangeability after working and corrosion resistance after painting. The steel sheet contains, on the basis of mass percent, C: 0.02% to 0.20%, Si: 0.3% or less, Mn: 0.5% to 2.5%, P: 0.06% or less, S: 0.01% or less, Al: 0.1% or less, Ti: 0.05% to 0.25%, and V: 0.05% to 0.25%, the remainder being Fe and incidental impurities. The steel sheet has a substantially ferritic single phase, the ferritic single phase containing precipitates having a size of less than 20 nm, the precipitates containing 200 to 1750 mass ppm Ti and 150 to 1750 mass ppm V, V dissolved in solid solution being 200 or more but less than 1750 mass ppm.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2008/064175, with an international filing date of Jul. 31, 2008 (WO 2009/017256 A1, published Feb. 5, 2009), which is based on Japanese Patent Application No. 2007-198944, filed Jul. 31, 2007, the subject matter of which is incorporated by reference.

TECHNICAL FIELD

This disclosure relates to a high-strength steel sheet having high stretch flange-ability after working and corrosion resistance after painting.

BACKGROUND

Automobile parts, such as chassis and truck frames, require formability (mainly elongation and stretch flangeability), and steel having a tensile strength on the order of 590 MPa has been used for such applications. However, to reduce the effects of automobiles on the environment and to improve crashworthiness of automobiles, use of higher-strength automotive steel sheets has been promoted in recent years, and use of steel having a tensile strength on the order of 780 MPa is being investigated.

In general, steel materials having higher strength have lower workability. High-strength high-workability steel sheets have therefore been studied. For example, Japanese Patent No. 3591502 B2 and Japanese Unexamined Patent Application Publication Nos. 2006-161112 A, 2004-143518 A, 2003-321740 A, 2003-293083 A and 2003-160836 A describe techniques for improving elongation and stretch flangeability.

Japanese Patent No. 3591502 B2 discloses a technique relating to high-workability high-strength steel sheet having a tensile strength of 590 MPa or more, wherein the steel sheet has a substantially ferritic single phase in which carbide containing Ti and Mo having an average particle size of less than 10 nm is dispersedly precipitated.

Japanese Unexamined Patent Application Publication No. 2006-161112 A discloses a technique relating to a high-strength hot-rolled steel sheet having a strength of 880 MPa or more and a yield ratio of 0.80 or more. The steel sheet has a steel structure that contains, on the basis of mass, C: 0.08% to 0.20%, Si: 0.001% or more but less than 0.2%, Mn: more than 1.0% but not more than 3.0%, Al: 0.001% to 0.5%, V: more than 0.1% but not more than 0.5%, Ti: 0.05% or more but less than 0.2%, and Nb: 0.005% to 0.5%, provided that the following three formulae are satisfied, the remainder being Fe and incidental impurities, and that contains 70% by volume or more ferrite having an average particle size of 5 μm or less and a hardness of 250 Hv or more.

(Ti/48+Nb/93)×C/12≦4.5×10⁻⁵   (Formula 1)

0.5≦(V/51+Ti/48+Nb/93)/(C/12)≦5 1.5   (Formula 2)

V+Ti×2+Nb×1.4+C×2+Mn×0.1≧0.80   (Formula 3)

Japanese Unexamined Patent Application Publicatio No. 2004-143518 A discloses a technique relating to a hot-rolled steel .sheet that contains, on the basis of mass, C: 0.05% to 0.2%, Si: 0.001% to 3.0%, Mn: 0.5 to 3.0, P: 0.001% to 0.2%, Al: 0.001% to 3%, and V: more than 0.1% but not more than 1.5%, the remainder being Fe and impurities, and has a structure mainly composed of ferrite phase having an average particle size in the range of 1 to 5 μm, the ferrite particles containing carbonitride of V having an average particle size of 50 nm or less.

Japanese Unexamined Patent Application Publication No. 2003-321740 A discloses a thermally stable high-strength thin steel sheet that contains precipitated carbide in the steel structure. In the thin steel sheet, carbide has a NaCl-type crystal structure represented by MC wherein M denotes a metallic element composed of at least two metals, and the at least two metals are regularly spaced in a crystal lattice, forming a superlattice.

Japanese Unexamined Patent Application Publication No. 2003-293083 A discloses the following hot-rolled steel sheet. The steel sheet has a composition of C: 0.0002% to 0.25%, Si: 0.003% to 3.0%, Mn: 0.003% to 3.0%, and Al: 0.002% to 2.0% on the basis of mass percent, the remainder being Fe and incidental impurities, the impurities containing 0.15% or less P, 0.05% or less S, and 0.01% or less N. A ferrite phase accounts for 70% by area or more of the metal structure and has an average grain size of 20 μm or less and an aspect ratio of 3 or less. Seventy percent or more of ferrite grain boundaries are high-angle grain boundaries. Among ferrite phases defined by high-angle grain boundaries, the area percentage of precipitates having a maximum diameter of 30 μm or less and a minimum diameter of 5 nm or more is 2% or less of the metal structure. Second phases having the largest area percentage among phases other than the ferrite phases and the precipitates have an average grain size of 20 μm or less. High-angle grain boundaries of ferrite phases are disposed between the nearest second phases.

Japanese Unexamined Patent Application Publication No. 2003-160836 A discloses a drawable high-strength thin steel sheet that has excellent shape fixability and burring characteristics, wherein the thin steel sheet contains, on the basis of mass percent, C: 0.01% to 0.1%, S≦0.03%, N≦0.005%, and Ti: 0.05% to 0.5%, the Ti content satisfying Ti-48/12C-48/14N-48/32S≧0%, the remainder being Fe and incidental impurities, at least the mean values of X-ray random intensity ratios in a plane at half the thickness of the steel sheet are 3 or more for {100}<011> to {223}<110> orientations and 3:5 or less for three orientations of {554}<225>, {111}<112>, and {111}<110>, the arithmetical mean rough Ra of at least one of the surfaces of the steel sheet ranges from 1 to 3.5, and the steel sheet is coated with a lubricating composition.

However, the art described above has the following problems.

Because the steel sheet contains Mo in Japanese Patent No. 3591502 B2 and Japanese Unexamined Patent Application Publication No. 2003-321740 A, a recent increase in the cost of Mo has resulted in a marked increase in the cost of the steel sheet.

With the increasing globalization of the automobile industry, automotive steel sheets are being used under severe corrosion conditions and, therefore, steel sheets require higher corrosion resistance after painting. However, the addition of Mo prevents the formation or growth of crystals during chemical conversion, thereby lowering the corrosion resistance of a steel sheet after painting. The addition of Mo therefore cannot satisfy this requirement. Thus, the steel described in Japanese Patent No. 3591502 B2 and Japanese Unexamined Patent Application Publication No. 2003-321740 A does not have corrosion resistance after painting that satisfies recent requirements of the automobile industry.

With recent advances in pressing techniques, processing such as drawing or stretch forming→piercing→flange forming is increasingly employed. Flanges of steel sheets formed by such processing require stretch flangeability after drawing or stretch forming and piercing, that is, stretch flangeability after working. However, in Japanese Unexamined Patent Application Publication Nos. 2006-161112 A, 2004-143518 A and 2003-321740 A, a TS of 780 MPa or more is not always compatible with sufficient stretch flangeability after working. The addition of Nb in Japanese Unexamined Patent Application Publication No. 2004-143518 A significantly retards the recrystallization of austenite after hot rolling. Deformed austenite therefore remains in a steel sheet, thereby lowering workability. The addition of Nb also disadvantageously increases rolling load in hot rolling.

Japanese Unexamined Patent Application Publication No. 2003-293083 A discloses single-phase ferritic steel sheets having a tensile strength TS of 422 MPa or less (for example, test numbers 1 to 5 in Table. 6 and test number 45 in Table 8 in Examples) and multiphase steel sheets composed of a ferrite phase and a second phase and having a tensile strength TS of 780 MPa or more (for example, test numbers 33 to 36 in Table 6 and test number 49 in Table 8 in Examples). These steel sheets described in Japanese Unexamined Patent Application Publication No. 2003-293083 A mainly take advantage of solid-solution strengthening due to Si or Mn and transformation hardening utilizing a hard second phase. These steel sheets must therefore be cooled to a temperature in the range of 600° C. to 800° C. at an average cooling rate of 30° C./s or more within two seconds after finish rolling, air-cooled for 3 to 15 seconds, and then water-cooled at an average cooling rate of 30° C./s or more before coiling. This promotes two-phase separation during ferrite transformation, allowing the steel sheets to have a mixed structure of the ferrite phase and the second phase. The finish-rolling temperature ranges from (Ae3 point+100° C.) to Ae3 point, which is lower than the temperature range suitable for manufacture described below. For example, the finish-rolling temperature for multiphase steel sheets having a tensile strength TS of 780 MPa or more (test numbers 33 to 36 in Table 6 in Examples) ranged from 871° C. to 800° C. A low finish-rolling temperature results in a decrease in the solubility limit of a carbide-forming element, such as Ti, in an austenite phase. Furthermore, because rolling introduces precipitation sites, precipitates having a size of 20 nm or more are formed. This phenomenon is referred to as strain-induced precipitation. In the steel sheets and the method for manufacturing the steel sheets described in Japanese Unexamined Patent Application Publication No. 2003-293083, strain-induced precipitation increases the amount of precipitates having a size of 20 run or more.

Japanese Unexamined Patent Application Publication No. 2003-293083 A also discloses a technique in which a ferritic single phase can be manufactured by greatly decreasing the C content and decreasing the amount of austenite forming element, Mn, in a steel composition (see steel numbers AA to AE in Table 2 in Examples). However, a decrease in the amount of Mn, which is also a solid-solution strengthening element, lowers the solid-solution strengthening level. A decrease in C content results in a decrease in the amount of precipitated carbide, for example, of Ti or Nb, which has precipitation hardening effects, thereby lowering the precipitation hardening level. Thus, even with a combination of the solid-solution strengthening level and the precipitation hardening level, a single-phase ferritic steel sheet cannot have a strength of 780 MPa or more (see test numbers 1 to 5 in Table 6 and test number 45 in Table 8 in Examples). For those reasons, a steel sheet that has a substantially ferritic single phase, a tensile strength of 780 MPa or more, and other characteristics cannot be manufactured by the technique described in Japanese Unexamined Patent Application Publication No. 2003-293083 A.

Japanese Unexamined Patent Application Publication No. 2003-160836 A discloses steel sheets having a tensile strength σ_(B) of 780 MPa or more (for example, steel symbols A-4, A-8, A-10, C, E, and H in Table 2 in Examples). The YRs of these steel sheets (YR represents σ_(Y)/σ_(B)×100 (%)) are as low as 69% to 74%, indicating that these steel sheets contain a hard second phase, such as a martensite phase.

As in Japanese Unexamined Patent Application Publication No. 2003-293083 A, the possible basic ideas behind the design of a steel sheet having a strength of 780 MPa or more according to Japanese Unexamined Patent Application Publication No. 2003-160836 A mainly take advantage of solid-solution strengthening due to Si or Mn and transformation hardening utilizing a hard second phase. As described in Japanese Unexamined Patent Application Publication No. 2003-293083 A, therefore, rolling at a total reduction of 25% or more must be performed at a finish-rolling temperature (Ar3 point+100° C. or less) lower than the temperature range suitable for manufacture. For example, according to an example of Japanese Unexamined Patent Application Publication No. 2003-160836 A, the finish-rolling temperature for a steel sheet having a tensile strength σ_(B) of 780 MPa or more ranged from 800° C. to 890° C. In the steel sheets and the method for manufacturing the steel sheets described in Japanese Unexamined Patent Application Publication No. 2003-160836 A, as described in Japanese Unexamined Patent Application Publication No. 2003-293083 A, strain-induced precipitation increases the amount of precipitates having a size of 20 nm or more. Consequently, a steel sheet that has a substantially ferritic single phase, a tensile strength of 780 MPa or more, and other characteristics cannot be manufactured.

In view of the situations described above, it could be helpful to provide a high-strength steel sheet having high stretch flangeability after working and corrosion resistance after painting.

SUMMARY

As a result of investigations to develop a high-strength hot-rolled steel sheet that has high stretch flangeability after working, corrosion resistance after painting, and a tensile strength of 780 MPa or more, we found as follows:

-   -   i) To manufacture a high-strength steel sheet having high         corrosion resistance after painting, precipitates must remain         fine (less than 20 nm), and the percentage of fine precipitates         (having a size less than 20 nm) must be increased. Although         precipitates containing Ti—Mo or Ti—V remain fine, mixed         precipitation of Ti and V is useful in improving corrosion         resistance after painting; and     -   ii) Solid solution of V is important in improving stretch         flangeability after working. There is an optimum V content of         solid solution for an improvement in characteristics.

We thus provide:

-   -   [1] A high-strength steel sheet containing, on the basis of mass         percent, C: 0.02% to 0.20%, Si: 0.3% or less, Mn: 0.5% to 2.5%,         P: 0.06% or less, S: 0.01% or less, Al: 0.1% or less, Ti: 0.05%         to 0.25%, and V: 0.05% to 0.25%, the remainder being Fe and         incidental impurities, wherein the steel sheet has a         substantially ferritic single phase, the ferritic single phase         containing precipitates having a size of less than 20 nm, the         precipitates containing 200 to 1750 mass ppm Ti and 150 to 1750         mass ppm V, V dissolved in solid solution being 200 or more but         less than 1750 mass ppm.     -   [2] In [1], the steel sheet further contains, on the basis of         mass percent, any one or two or more of Cr: 0.01% to 0.5%, W:         0.005% to 0.2%, and Zr: 0.0005% to 0.05%.     -   [3] In [1] or [2], the high-strength steel sheet has a tensile         strength TS of 780 MPa or more.     -   [4] In [1] or [2], the high-strength steel sheet has a one-side         maximum peel width of 3.0 mm or less after a tape peel test in a         warm salt water immersion test.     -   [5] In [3], the high-strength steel sheet has a one-side maximum         peel width of 3.0 mm or less after a tape peel test in a warm         salt water immersion test.     -   [6] In [1] or [2], the high-strength steel sheet has a stretch         flangeability λ₁₀ of 60% or more after rolling at an elongation         percentage of 10%.     -   [7] In [3], the high-strength steel sheet has a stretch         flangeability λ₁₀ of 60% or more after rolling at an elongation         percentage of 10%.

DETAILED DESCRIPTION

The percentages and ppm of components of steel are based on mass percent and mass ppm. Our high-strength steel sheets have a tensile strength (hereinafter also referred to as TS) of 780 MPa or more and include hot-rolled steel sheets and surface-treated steel sheets, which are high-strength steel sheets subjected to surface treatment, such as plating.

Target characteristics include a stretch flangeability (λ₁₀) of 60% or more after rolling at an elongation percentage of 10% and a one-side maximum peel width of 3.0 mm or less after a tape peel test in a warm salt water immersion test (SDT) described below.

We provide a high-strength hot-rolled steel sheet that has high stretch flange-ability after working, corrosion resistance after painting, and a TS of 780 MPa or more. Our steel sheets have these advantages without the addition of Mo and can therefore reduce costs.

For example, use of a high-strength hot-rolled steel sheet in automobile chassis and truck frames should allow thickness reduction, reduce the effects of automobiles on the environment, and markedly improve crashworthiness of automobiles.

Our steel sheets and methods will be described in detail below.

(1) First, the reason to specify the chemical components (composition) of steel will be described below.

C: 0.02% to 0.20%

C can be precipitated in ferrite as carbide with Ti or V, thereby contributing to high strength of a steel sheet. 0.02% or more C is required to achieve a TS of 780 MPa or more. However, more than 0.20% C results in coarsening of precipitates and the formation of a second phase, lowering stretch flangeability after working. Thus, the C content ranges from 0.02% to 0.20%, preferably 0.03% to 0.15%.

Si: 0.3% or Less

Although Si can contribute to solid-solution strengthening, the addition of more than 0.3% Si results in the formation of cementite at grain boundaries, lowering stretch flangeability after working. Thus, the Si content is 0.3% or less, preferably 0.001% to 0.2%.

Mn: 0.5% to 2.5%

Mn can contribute to solid-solution strengthening. However, the TS is less than 780 MPa at a Mn content of less than 0.5%. The addition of more than 2.5% Mn markedly lowers weldability. Thus, the Mn content ranges from 0.5% to 2.5%, preferably 0.6% to 2.0%.

P: 0.06% or Less

P can segregate at prior austenite grain boundaries, lowering workability and low-temperature toughness. Thus, the P content is preferably minimized and is 0.06% or less, preferably in the range of 0.001% to 0.055%.

S: 0.01% or Less

S can segregate at prior austenite grain boundaries or can be precipitated as MnS. The segregation or a large amount of MnS lowers low-temperature toughness. S also markedly lowers stretch flangeability, regardless of the presence or absence of working. Thus, the S content is preferably minimized and is 0.01% or less, preferably in the range of 0.0001% to 0.005%.

Al: 0.1% or Less

Al can be added to steel as a deoxidizer and effectively improves the cleanliness of the steel. Preferably, 0.001% or more Al is added to steel to produce this effect. However, more than 0.1% Al results in the generation of a large number of inclusions, causing flaws in a steel sheet. Thus, the Al content is 0.1% or less, preferably 0.01% to 0.04%.

Ti: 0.05% to 0.25%

Ti is very important for the precipitation hardening of ferrite and is an important factor in our steel sheets. A required strength is difficult to achieve at a Ti content of less than 0.05%. However, the effects of Ti become saturated at a Ti content of more than 0.25%, and more than 0.25% Ti only increases costs. Thus, the Ti content ranges from 0.05% to 0.25%, preferably 0.08% to 0.20%.

V: 0.05% to 0.25%

V can contribute to an improvement in strength by precipitation hardening or solid-solution strengthening. Like Ti, V is therefore an important factor in our steel sheets. A proper amount of V, together with Ti, tends to be precipitated as fine Ti-V carbide having a particle size (hereinafter also referred to as “size”) of less than 20 nm. Unlike Mo, V does not lower corrosion resistance after painting. Less than 0.05% V is insufficient for the effects described above. However, the effects of V become saturated at a V content of more than 0.25%, and more than 0.25% V only increases costs. Thus, the V content ranges from 0.05% to 0.25%, preferably 0.06% to 0.20%.

With these essential additive elements, the steels can have target characteristics. In addition to the essential additive elements, any one or two or more of Cr: 0.01% to 0.5%, W: 0.005% to 0.2%, and Zr: 0.0005% to 0.05% may be added for the following reasons. Cr: 0.01% to 0.5%, W: 0.005% to 0.2%, and Zr: 0.0005% to 0.05%

Like V, Cr, W, and Zr can strengthen ferrite as a precipitate or solid solution. Less than 0.01% Cr, less than 0.005% W, or less than 0.0005% Zr makes a negligible contribution to high strength of steel. However, more than 0.5% Cr, more than 0.2% W, or more than 0.05% Zr lowers workability. Thus, when any one or two or more of Cr, W, and Zr are added, their amounts are Cr: 0.01% to 0.5%, W: 0.005% to 0.2%, and Zr: 0.0005% to 0.05%, preferably Cr: 0.03% to 0.3%, W: 0.01% to 0.18%, and Zr: 0.001% to 0.04%.

The remainder consists of Fe and incidental impurities. As an incidental impurity, for example, ◯ forms a non-metallic inclusion and has adverse effects on the quality of steel. ◯ is therefore desirably decreased to 0.003% or less. 0.1% or less Cu, Ni, Sn, and/or Sb may be contained as a trace element without compromising the operational advantages of our steel sheets.

(2) The structure of a high-strength steel sheet will be described below.

Substantially Ferritic Single Phase

To achieve a TS of 780 MPa or more and improve stretch flangeability after working, ferrite having a low dislocation density is effective, and a single phase is effective. In particular, a highly ductile ferritic single phase has a marked improving effect on stretch flangeability after working. However, a completely ferritic single phase is not necessary, and even a substantially ferritic single phase can sufficiently produce the effect. A substantially ferritic single phase, as used herein, refers to allowance for a minute amount of another phase or precipitate other than carbide, and the volume percentage of ferrite is preferably 95% or more. A substantially ferritic single phase may contain up to 5% by volume of cementite, pearlite, and/or bainite without affecting the characteristics of the steel sheets.

The volume percentage of ferrite can be determined by exposing a microstructure in the vertical cross-section parallel to the rolling direction using 3% nital, observing the microstructure at a quarter thickness in the depth direction with a scanning electron microscope (SEM) at a magnification of 1500, and determining the ferrite area ratio, for example, using an image-processing software “Ryusi Kaiseki (particle analysis) II” from Sumitomo Metal Technology, Inc.

200 to 1750 ppm Ti and 150 to 1750 ppm V in Precipitates Having a Size below 20 nm in a Ferritic Single Phase

In a high-strength steel sheet, precipitates containing Ti and/or V exist in ferrite mainly as carbides. This is probably because the solubility limit of C in ferrite is low, and supersaturated C is therefore easily precipitated in ferrite as carbide. Such a precipitate increases the hardness (strength) of soft ferrite, thereby achieving a TS of 780 MPa or more. Such a precipitate also increases YS, achieving YR (═YS/YR) of 83% or more.

As described above, to manufacture a high-strength steel sheet, it is important that precipitates remain fine (less than 20 nm), and the percentage of fine precipitates (having a size less than 20 nm) is increased. A precipitate having a size of 20 nm or more has a small effect in preventing dislocation movement and cannot sufficiently increase the hardness of ferrite, sometimes resulting in low strength.

A further investigation revealed that a fine precipitate size is important for corrosion resistance after painting. In conventional Ti (addition of Ti alone) HSLA steel, a precipitate have a tendency to become coarse with increasing Ti content. In such a steel sheet, therefore, corrosion resistance after painting also has a tendency to decrease with decreasing strength. Although the reason for a deterioration in corrosion resistance after painting associated with coarsening of a precipitate is not clear, a coarse precipitate should prevent the formation or growth of crystals during chemical conversion.

Thus, a precipitate preferably has a size of less than 20 nm. A fine precipitate having a size of less than 20 nm can be formed by the addition of both Ti and V. V forms a complex carbide mainly with Ti. Although there is no clear reason, these precipitates remain stable and fine at high temperatures within the coiling temperature for a long period of time.

It is important to control the Ti content and the V content of precipitates having a size of less than 20 nm. When the Ti content and the V content of precipitates having a size of less than 20 nm are less than 200 ppm and less than 150 ppm, respectively, the number density of the precipitates is small, and the distance between precipitates increases. The precipitates therefore have a small effect in preventing dislocation movement. Thus, the precipitates cannot sufficiently increase the hardness of ferrite, and therefore the TS cannot be 780 MPa or more. When the Ti content and the V content of precipitates having a size of less than 20 nm are 200 ppm or more and less than 150 ppm, respectively, the precipitates have a tendency to become coarse, and therefore the TS may be less than 780 MPa. When the Ti content and the V content of precipitates having a size of less than 20 nm are less than 200 ppm and 150 ppm or more, respectively, the precipitation efficiency of V decreases, and therefore the TS may be less than 780 MPa. When the Ti content or the V content of precipitates having a size of less than 20 nm is more than 1750 ppm, the corrosion resistance after painting decreases, and therefore the target characteristics cannot be achieved. This is probably because a large number of fine precipitates prevent the formation or growth of crystals on the surface of a steel sheet during chemical conversion. Thus, the amounts of precipitated Ti and V in precipitates having a size of less than 20 nm must be satisfactorily controlled.

When the ratio of the Ti content to the V content of precipitates having a size of less than 20 nm satisfies 0.4≦(Ti/48)/(V/51)≦2.5, the TS can be 785 MPa or more, thus achieving more suitable conditions. Although there is no clear reason, optimization of the ratio of Ti to V should improve heat stability.

Thus, the Ti content and the V content of precipitates having a size of less than 20 nm range from 200 to 1750 ppm and 150 to 1750 ppm, respectively. Furthermore, the ratio of the Ti content to the V content of precipitates having a size of less than 20 nm preferably satisfies 0.4≦(Ti/48)/(V/51)≦2.5.

A precipitate and/or an inclusion is hereinafter also collectively referred to as a precipitate or the like.

The Ti content and the V content can be controlled by the coiling temperature. The coiling temperature preferably ranges from 500° C. to 700° C. At a coiling temperature above 700° C., precipitates become coarse, and the amounts of precipitated Ti and V in precipitates having a size of less than 20 nm are less than 200 ppm and less than 150 ppm, respectively, and the TS cannot be 780 MPa or more. At a coiling temperature below 500° C., the amounts of precipitated Ti and V in precipitates having a size of less than 20 nm are also less than 200 ppm and less than 150 ppm, respectively. Such a low coiling temperature should result in insufficient diffusion of Ti and V.

The Ti content and the V content of precipitates having a size of less than 20 nm can be determined by the following method.

After a predetermined amount of sample is electrolyzed in an electrolyte, the sample is removed from the electrolyte and is immersed in a dispersive solution. Precipitates in the solution is filtered with a filter having a pore size of 20 nm. Precipitates in filtrate passing through the filter having a pore size of 20 nm have a size of less than 20 nm. The filtrate after filtration is appropriately analyzed by inductively coupled plasma (ICP) emission spectroscopic analysis, ICP mass spectrometry, atomic absorption spectrometry, or the like to determine the Ti content and the V content of precipitates having a size of less than 20 nm. Structure Containing 200 ppm or More but Less Than 1750 ppm V in Solid Solution.

V in solid solution is the most important factor. Solid solution of V is important in improving stretch flangeability after working. Less than 200 ppm V in solid solution has an insufficient effect, and 200 ppm or more V in solid solution is required to produce the effect described above. 1750 ppm or more V in solid solution exhibits a saturated effect and is considered as an upper limit.

Thus, the amount of V in solid solution is 200 ppm or more but less than 1750 ppm. Although the workability of steel slightly deteriorates with increasing strength, when the Ti content and the V content of precipitates having a size of less than 20 nm are both 1750 ppm or less, 200 ppm or more V in solid solution can sufficiently ensure target stretch flangeability after working.

200 ppm or more but less than 1750 ppm V in solid solution can be measured, for example, by the following method.

After a predetermined amount of sample is electrolyzed in a nonaqueous solvent electrolyte, the electrolyte is subjected to elementary analysis. The analysis method may be inductively coupled plasma (ICP) emission spectroscopic analysis, ICP mass spectrometry, or atomic absorption spectrometry.

(3) A method for manufacturing a high-strength steel sheet will be described below.

For example, a high-strength steel sheet can be manufactured by heating a steel slab adjusted within the chemical component ranges described above at a temperature in the range of 1150° C. to 1350° C., hot-rolling the steel slab at a finish-rolling temperature in the range of 850° C. to 1100° C., and coiling the rolled steel at a temperature in the range of 500° C. to 700° C. Conditions suitable for these processes will be described in detail below.

Steel Slab Heating Temperature: 1150° C. to 1350° C.

A carbide-forming element, such as Ti or V, is mostly present as a precipitate in a steel slab. To be precipitated as desired in a ferrite phase after hot rolling, a precipitate in the form of carbide must be temporarily dissolved before hot rolling. A precipitate must therefore be heated at 1150° C. or more.

At a temperature below 1150° C., carbide having a size of 20 nm or more, which does not contribute to precipitation hardening or corrosion resistance after painting, remains. This reduces the amount of Ti and V involved in the formation of fine precipitates having a size of less than 20 nm required. A target amount of precipitates having a size of less than 20 nm cannot therefore be obtained in coiling described below. In a method for manufacturing a steel sheet, most desirably, carbide containing Ti or V remains dissolved during slab heating and finish rolling, and is precipitated as fine carbide containing Ti or V during coiling after finish rolling. The heating temperature is therefore more preferably 1170° C. or more so that carbide can be dissolved almost completely.

However, heating at a temperature above 1350° C. excessively increases the crystal grain size, lowering stretch flangeability and elongation after working. Taking subsequent heat-treatment conditions into consideration, an increase in crystal grain size can be almost completely prevented at a heating temperature of 1300° C. or less.

Thus, the slab heating temperature preferably ranges from 1150° C. to 1350° C., more preferably 1170° C. to 1300° C.

Finish-Rolling Temperature in Hot Rolling: 850° C. to 1100° C.

The control of finish-rolling temperature is important in ensuring the Ti content and the V content of precipitates having a size of less than 20 nm. Preferably, a steel slab after working is hot-rolled at a finish-rolling temperature in the range of 850° C. to 1100° C., which is the final temperature of hot rolling. At a finish-rolling temperature below 850° C., a steel slab is rolled in a ferrite+austenite region and has an elongated ferrite phase. This may lower stretch flangeability or elongation after working. Even if a steel slab is heated at a temperature of 1150° C. or more to temporarily dissolve a carbide precipitate before rolling, carbide containing Ti or V is precipitated at a finish-rolling temperature below 850° C. because of strain-induced precipitation. This reduces the amount of Ti and V involved in the formation of fine precipitates having a size of less than 20 nm required. A target amount of precipitates having a size of less than 20 nm cannot therefore be obtained in coiling described below. Thus, it is important to perform the subsequent coiling process while carbide containing Ti or V temporarily dissolved during the slab heating described above remains dissolved in finish rolling as much as possible. The finish-rolling temperature is more preferably 935° C. or more such that carbide remains dissolved.

A finish-rolling temperature above 1100° C. may result in coarsening of ferrite particles and a TS below 780 MPa. The finish-rolling temperature is more preferably 990° C. or less to prevent coarsening of ferrite particles.

Thus, the finish-rolling temperature preferably ranges from 850° C. to 1100° C., more preferably 935° C. to 990° C.

Coiling Temperature: 500° C. to 700° C.

The control of coiling temperature is important in ensuring the Ti content and the V content of precipitates having a size of less than 20 nm. As described above, this is because, in the most desirable manufacturing form, this coiling process yields a large number of precipitation sites from which carbide is precipitated, thus preventing carbide grains from growing to 20 nm or more. The coiling temperature preferably ranges from 500° C. to 700° C. so that steel has a substantially ferritic single phase and the characteristics can be achieved.

A coiling temperature below 500° C. may result in an insufficient amount of precipitated carbide containing Ti and/or V and reduced strength. Furthermore, a bainite phase may be formed in place of a ferritic single phase.

To form a large number of precipitation sites and produce carbide from these precipitation sites, the coiling temperature is preferably 500° C. or more, more preferably 550° C. or more.

A coiling temperature above 700° C. may result in coarsening of precipitated carbide and reduced strength. A coiling temperature above 700° C. may also promote the formation of a pearlite phase, lowering stretch flangeability after working. The coiling temperature is more preferably 650° C. or less to prevent coarsening of precipitated carbide without fail.

Thus, the coiling temperature preferably ranges from 500° C. to 700° C., more preferably 550° C. to 650° C.

The steel sheets include surface-treated steel sheets and surface-coated steel sheets. In particular, a steel sheet may be subjected to hot-dip galvanizing to form a galvanized steel sheet, and this disclosure can be suitably applied to such a galvanized steel sheet. Because our steel sheets have excellent workability, such a galvanized steel sheet can also have excellent workability. Hot-dip galvanizing is zinc and zinc-based (approximately 90% or more) hot dipping and includes hot dipping including an alloying element, such as Al or Cr, as well as zinc. Hot-dip galvanizing may be performed alone or followed by alloying.

A steel melting method is not particularly limited, and any known melting method may be suitable. For example, a suitable melting method involves melting in a converter or an electric furnace and secondary refining in a vacuum degassing furnace. A casting method is preferably continuous casting in terms of productivity and quality. After casting, hot direct rolling may be performed immediately or after concurrent heating, without compromising the advantages of our steel sheets. Furthermore, a hot-rolled material may be heated after rough rolling and before finish rolling, continuous hot rolling in which rolled materials are joined may be performed after rough rolling, or heating and continuous rolling of a heating material of a rolled material may be performed simultaneously. These do not compromise the advantages of our steel sheets.

Examples Example 1

Steel having a composition shown in Table 1 was melted in a converter and was formed into a steel slab by continuous casting. The steel slab was subjected to heating, hot rolling, and coiling under conditions shown in Table 2 to form a hot-rolled steel sheet having a thickness of 2.0 mm.

TABLE 1 Type of Composition (mass %) steel C Si Mn P S Al Ti V Note A 0.040 0.01 1.45 0.01 0.0015 0.03 0.105 0.120 Conforming steel B 0.120 0.02 1.20 0.02 0.0008 0.03 0.240 0.100 Conforming steel C 0.100 0.02 1.20 0.01 0.0080 0.03 0.110 0.245 Conforming steel D 0.150 0.02 1.40 0.03 0.0020 0.03 0.230 0.224 Conforming steel E 0.050 0.01 2.02 0.01 0.0020 0.03 0.120 0.120 Conforming steel F 0.050 0.01 0.65 0.01 0.0015 0.03 0.110 0.136 Conforming steel G 0.045 0.02 1.34 0.02 0.0007 0.02 0.060 0.110 Conforming steel H 0.050 0.02 1.30 0.01 0.0008 0.02 0.110 0.052 Conforming steel I 0.030 0.01 1.32 0.01 0.0007 0.02 0.080 0.070 Conforming steel J 0.040 0.01 1.40 0.02 0.0015 0.03 0.126 0.152 Conforming steel K 0.250 0.01 1.20 0.02 0.0020 0.03 0.120 0.130 Nonconforming L 0.001 0.01 1.19 0.02 0.0020 0.03 0.120 0.130 Nonconforming M 0.080 0.50 1.30 0.01 0.0012 0.03 0.070 0.070 Nonconforming N 0.050 0.01 0.35 0.02 0.0015 0.03 0.080 0.080 Nonconforming O 0.050 0.01 3.00 0.02 0.0014 0.03 0.080 0.080 Nonconforming P 0.150 0.01 1.60 0.02 0.0015 0.03 0.040 0.120 Nonconforming Q 0.160 0.01 1.60 0.02 0.0016 0.02 0.070 0.032 Nonconforming R 0.152 0.01 1.62 0.02 0.0015 0.03 0.280 0.120 Nonconforming S 0.161 0.01 1.61 0.02 0.0014 0.03 0.150 0.300 Nonconforming X 0.090 0.06 1.35 0.04 0.0014 0.05 0.150 0.160 Conforming steel

The microstructure of the hot-rolled steel sheet was analyzed by the following method to determine the Ti content and the V content of precipitates having a size of less than 20 nm and the amount of V in solid solution. The tensile strength TS, the stretch flange-ability after working λ₁₀, and the corrosion resistance after painting (SDT one-side maximum peel width) were measured.

Analysis of Microstructure

The hot-rolled steel sheet thus formed was cut into an appropriate size. Approximately 0.2 g of hot-rolled steel sheet was subjected to constant-current electrolysis at an electric current density of 20 mA/cm² in 10% AA electrolyte (10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol).

Measurement of the Ti Content and the V Content of Precipitates Having a Size of Less Than 20 nm

After electrolysis, a test piece on which a precipitate was deposited was removed from the electrolyte and was immersed in aqueous sodium hexametaphosphate (500 mg/l) (hereinafter referred to as aqueous SHMP). Ultrasonic vibration was applied to the test piece to detach and extract the precipitate from the test piece in aqueous SHMP. The aqueous SHMP containing the precipitate was then passed through a filter having a pore size of 20 nm. The filtrate was analyzed with an ICP spectrometer to measure the absolute amounts of Ti and V in the filtrate. The absolute amounts of Ti and V were divided by the weight of the electrolyzed sample to calculate the Ti content and the V content of precipitates having a size of less than 20 nm. The weight of electrolyzed sample was calculated by subtracting the sample weight after the detachment of the precipitate from the sample weight before electrolysis.

Measurement of the Amount of V in Solid Solution

After electrolysis, the concentrations of V and a comparative element Fe in the electrolyte were measured by ICP mass spectrometry. On the Basis of the concentrations thus measured, the ratio of the concentration of V to the concentration of Fe was calculated. The ratio was multiplied by the Fe content of the sample to calculate the amount of V in solid solution. The Fe content of the sample can be calculated by subtracting the summation of compositions other than Fe from 100%.

TS

A tensile test according to JIS Z 2241 was performed with a JIS No. 5 specimen in the tensile direction parallel to the rolling direction to measure TS.

Stretch Flangeability after Working: λ₁₀

After rolling at an elongation percentage of 10%, a hole expanding test according to the Japan Iron and Steel Federation Standard JFS T 1001 was performed to measure λ₁₀.

Corrosion Resistance after Painting: SDT One-Side Maximum Peel Width

A chemical conversion treatment was performed under more adverse temperature and concentration conditions than the standard conditions using a degreasing agent, Surf-cleaner ECO90, a surface conditioner, Surffine 5N-10, and a chemical conversion treatment agent, Surfdine SD2800, all manufactured by Nippon Paint Co., Ltd. As an example of standard conditions, a degreasing process included a concentration of 16 g/l, a treatment temperature in the range of 42° C. to 44° C., a treatment time of 120 s, and spray degreasing, and a surface conditioning process included a total alkalinity in the range of 1.5 to 2.5 points, a free acidity in the range of 0.7 to 0.9 points, an accelerator concentration in the range of 2.8 to 3.5 points, a treatment temperature of 44° C., and a treatment time of 120 s. Under adverse conditions, a treatment temperature in a chemical conversion treatment process was decreased to 38° C. Subsequently, electrodeposition coating was performed using an electrodeposition paint, V-50, manufactured by Nippon Paint Co., Ltd. The target amount of deposited chemical conversion film ranged from 2 to 2.5 g/m², and the target film thickness in electrodeposition coating was 25 μm.

Corrosion resistance after painting was determined in a warm salt water immersion test (SDT). A crosscut was formed with a cutter in a sample subjected to chemical conversion treatment and electrodeposition coating. The sample was immersed in warm salt water (5% NaCl at 55° C.) for 10 days, was then washed with water, and was dried. Tape peeling on the crosscut was performed to measure the maximum peel width on the left and right sides of the crosscut. A one-side maximum peel width of 3.0 mm or less was considered as high corrosion resistance after painting.

Table 2 shows the results, together with manufacturing conditions.

TABLE 2 Elonga- Stretch Precip- Precip- Amount One- Slab Finish- tion flange- itated itated of V side heating rolling Coiling after ability Ti content V content in solid maximum Type temper- temper- temper- pre- after for <20 for <20 solution peel of ature ature ature TS straining working: nm (mass nm (mass (mass width No steel (° C.) (° C.) (° C.) (MPa) (%) λ10(%) ppm) ppm) ppm) (mm) Phase Note 1 A 1250 920 630 812 20 87 752 818 340 1.7 Ferrite: Example 100% 2 B 1300 926 632 952 18 79 1580 765 231 2.5 Ferrite: Example 100% 3 C 1270 911 650 966 17 81 703 1700 380 2.2 Ferrite: Example 100% 4 C 1270 900 580 865 17 95 635 657 1350 1.2 Ferrite: Example 99%, Remainder Cementite 1% 5 D 1270 917 603 1190 16 61 1700 1682 213 2.2 Ferrite: Example 98%, Remainder: Bainite 2% 6 E 1250 921 611 940 18 92 808 658 476 1.2 Ferrite: Example 100% 7 F 1250 900 590 834 20 98 727 735 540 1.4 Ferrite: Example 100% 8 G 1250 918 670 815 19 82 230 450 420 1.4 Ferrite: Example 100% 9 H 1250 920 580 802 18 93 352 167 272 1.2 Ferrite: Example 100% 10 I 1160 905 625 785 22 97 532 372 306 1.2 Ferrite: Example 100% 11 J 1250 920 630 936 18 83 863 1129 274 2.0 Ferrite: Example 100% 12 A 1250 920 480 760 19 63 150 121 934 2.0 Ferrite: Comparative 100% Example 13 G 1250 920 720 765 18 90 220 98 330 5.2 Ferrite: Comparative 100% Example 14 G 1250 915 750 760 15 78 140 80 908 5.1 Ferrite: Comparative 100% Example 15 K 1250 923 590 851 20 45 821 702 568 0.8 Ferrote” Comparative 90%, Example Remainder: Pearlite 10% 16 L 1250 918 585 659 25 60 50 45 568 1.1 Ferrite: Comparative 100% Example 17 M 1250 918 595 850 17 40 480 353 330 0.8 Ferrite: Comparative 92%, Example Remainder: Cementite 8% 18 N 1250 920 575 765 18 75 560 540 247 1.0 Ferrite: Comparative 100% Example 19 O 1250 916 565 851 14 43 560 432 350 1.1 Ferrite: Comparative 100% Example 20 P 1160 921 575 653 23 75 180 324 832 1.2 Ferrite: Comparative 100% Example 21 Q 1160 922 650 765 16 73 490 14 223 1.2 Ferrite: Comparative 100% Example 22 Q 1160 920 510 782 16 50 502 220 90 1.1 Ferrite: Comparative 100% Example 23 R 1250 910 605 1280 13 93 2065 602 580 5.5 Ferrite: Comparative 100% Example 24 S 1250 900 610 1290 14 91 971 1890 530 5.3 Ferrite: Comparative 100% Example 29 A 1250 935 600 825 19 70 800 825 340 2.0 Ferrite: Example 100% 30 A 1260 980 580 820 19 68 802 830 355 2.1 Ferrite: Example 100% 31 A 1260 1020 630 826 18 73 801 824 349 2.1 Ferrite: Example 100% 32 J 1260 940 620 982 17 63 923 1120 270 2.6 Ferrite: Example 100% 33 C 1260 960 600 983 17 65 812 1702 375 2.5 Ferrite: Example 100% 34 X 1300 965 600 1005 16 62 1205 1108 305 2.8 Ferrite: Example 100%

Table 2 shows that the working examples had a TS of 780 MPa or more, λ₁₀ of 60% or more, and an SDT one-side maximum peel width of 3.0 mm or less, indicating that the hot-rolled steel sheets had high stretch flangeability after working and corrosion resistance after painting.

In contrast, the comparative examples had a low TS (strength), small λ₁₀ (stretch flangeability after working), and/or a large SDT one-side maximum peel width (corrosion resistance after painting). Example 2

Steel having a composition shown in Table 3 was melted in a converter and was formed into a steel slab by continuous casting. The steel slab was subjected to heating, hot rolling, and coiling under conditions shown in Table 4 to form a hot-rolled steel sheet having a thickness of 2.0 mm.

TABLE 3 Type of Composition (mass %) steel C Si Mn P S Al Ti V Cr W Zr Note T 0.040 0.01 1.40 0.01 0.0014 0.03 0.100 0.115 0.10 — — Conforming steel U 0.040 0.02 1.43 0.01 0.0015 0.03 0.104 0.105 — 0.150 — Conforming steel V 0.041 0.01 1.42 0.01 0.0014 0.03 0.102 0.105 — — 0.0030 Conforming steel W 0.040 0.02 1.40 0.01 0.0014 0.03 0.101 0.115 0.20 0.140 0.0050 Conforming steel

In the same way as in Example 1, the microstructure of the hot-rolled steel sheet thus formed was analyzed to determine the Ti content and the V content of precipitates having a size of less than 20 nm and the amount of V in solid solution. In the same way as in Example 1, the tensile strength TS, the stretch flangeability after working λ₁₀, and the corrosion resistance after painting (SDT one-side maximum peel width) were measured.

Table 4 shows the results.

TABLE 4 Elonga- Stretch Precip- Precip- Amount One- Slab Finish- tion flange- itated itated of V side heating rolling Coiling after ability Ti content V content in solid maximum Type temper- temper- temper- pre- after for <20 for <20 solution peel of ature ature ature TS straining working: nm (mass nm (mass (mass width No steel (° C.) (° C.) (° C.) (MPa) (%) λ10(%) ppm) ppm) ppm) (mm) Phase Note 25 T 1250 921 625 832 17 99 750 815 250 2.5 Ferrite: Example 100% 26 U 1250 918 620 830 18 90 753 760 252 2.2 Ferrite: Example 100% 27 V 1250 920 621 829 17 93 753 770 250 2.0 Ferrite: Example 100% 28 W 1250 921 620 842 18 98 760 823 251 2.6 Ferrite: Example 100% 35 T 1250 940 600 835 18 92 780 820 240 2.2 Ferrite: Example 100% 36 T 1270 960 630 840 17 93 782 823 244 2.1 Ferrite: Example 100% 37 T 1300 980 620 837 18 95 788 830 245 2.3 Ferrite: Example 100%

Table 4 shows that the working examples had a TS of 780 MPa or more, λ₁₀ of 60% or more, and an SDT one-side maximum peel width of 3.0 mm or less, indicating that the hot-rolled steel sheets had high stretch flangeability after working and corrosion resistance after painting.

As compared with the steel sheet No. 1 (Table 2), the steel sheets Nos. 25 to 28 and 35 to 37, which further contained Cr, W, or Zr, had an improved TS.

INDUSTRIAL APPLICABILITY

Our steel sheets have high strength, high stretch flangeability after working, and high corrosion resistance after painting, and are therefore most suitable for, for example, automobile and truck frames, and components that require elongation and stretch flangeability. 

1. A high-strength steel sheet comprising, on the basis of mass percent, C: 0.02% to 0.20%, Si: 0.3% or less, Mn: 0.5% to 2.5%, P: 0.06% or less, S: 0.01% or less, Al: 0.1% or less, Ti: 0.05% to 0.25%, and V: 0.05% to 0.25%, the remainder being Fe and incidental impurities, having a substantially ferritic single phase, containing precipitates having a size of less than 20 nm, the precipitates containing 200 to 1750 mass ppm Ti and 150 to 1750 mass ppm V, V dissolved in solid solution being 200 or more but less than 1750 mass ppm.
 2. The high-strength steel sheet according to claim 1, further comprising, on the basis of mass percent, any one or two or more of Cr: 0.01% to 0.5%, W: 0.005% to 0.2%, and Zr: 0.0005% to 0.05%.
 3. The high-strength steel sheet according to claim 1, having a tensile strength TS of 780 MPa or more.
 4. The high-strength steel sheet according to claim 1, having a one-side maximum peel width of 3.0 mm or less after a tape peel test in a warm salt water immersion test.
 5. The high-strength steel sheet according to claim 3, having a one-side maximum peel width of 3.0 mm or less after a tape peel test in a warm salt water immersion test.
 6. The high-strength steel sheet according to claim 1, having a stretch flangeability λ₁₀ of 60% or more after rolling at an elongation percentage of 10%.
 7. The high-strength steel sheet according to claim 3, having a stretch flangeability λ ₁₀ of 60% or more after rolling at an elongation percentage of 10%.
 8. The high-strength steel sheet according to claim 2, having a tensile strength TS of 780 MPa or more.
 9. The high-strength steel sheet according to claim 2, having a one-side maximum peel width of 3.0 mm or less after a tape peel test in a warm salt water immersion test.
 10. The high-strength steel sheet according to claim 8, having a one-side maximum peel width of 3.0 mm or less after a tape peel test in a warm salt water immersion test.
 11. The high-strength steel sheet according to claim 2, having a stretch flangeability λ₁₀ of 60% or more after rolling at an elongation percentage of 10%.
 12. The high-strength steel sheet according to claim 3, having a stretch flangeability λ₁₀ of 60% or more after rolling at an elongation percentage of 10%. 